1. Field of the Invention
This invention relates to an oxygen sensor deterioration-detecting system for an internal combustion engine which has oxygen sensors arranged in the exhaust system, respectively, at locations upstream and downstream of catalytic converters arranged therein, and more particularly to an oxygen sensor deterioration-detecting system for detecting deterioration of the upstream oxygen sensors which are used in carrying out air-fuel ratio control per cylinder group of the engine, such as a V-type engine.
2. Prior Art
In internal combustion engines in general, to control the air-fuel ratio of an air-fuel mixture supplied to the engine to a desired value, an oxygen sensor (hereinafter referred to as "the upstream O2 sensor") is provided in the exhaust system at a location upstream of a catalytic converter arranged therein, for detecting the concentration of oxygen present in exhaust gases, to thereby control the air fuel ratio of the mixture, based on an output from the O2 sensor.
An O2 sensor of this kind is liable to change in characteristics (internal resistance, electromotive force, and response time), due to its heat deterioration or the like. The use of an O2 sensor which has deteriorated characteristics adversely affects the accuracy of the air-fuel ratio control.
To overcome this inconvenience, various proposals have been made, which include a method of additionally providing an O2 sensor in the exhaust system at a location downstream of the catalytic converter (hereinafter referred to as the "downstream O2 sensor"), in order to correct undesirable changes in characteristics of the air-fuel ratio feedback control due to the output from the upstream O2 sensor, whereby the air-fuel ratio feedback control can be carried out with high accuracy. According to this proposal, in controlling the air-fuel ratio of the mixture supplied to the engine to a desired value in a feedback manner responsive to the output from the upstream O2 sensor, a control amount used in the air-fuel ratio feedback control is corrected based on an output from the downstream O2 sensor, to thereby compensate for deviation of the controlled air-fuel ratio from a proper value, due to deterioration of the upstream O2 sensor. However, this proposed method has a disadvantage that when the upstream O2 sensor has become heavily deteriorated beyond the limit of the above-mentioned compensation, it will result in degraded exhaust emission characteristics of the engine.
To overcome this disadvantage, the present assignee has already proposed, e.g. by Japanese Patent Application No. 4-225284, a method which includes controlling the air-fuel ratio of a mixture supplied to an internal combustion engine in a feedback manner responsive to outputs from the upstream and downstream O2 sensors, and then calculating, e.g. a period of inversion of the output from the upstream O2 sensor, to detect deterioration of the upstream O2 sensor, based on the calculated inversion period.
The above proposed method is applied to an engine in which a single O2 sensor is provided in the exhaust system at a location upstream of a catalytic converter arranged therein, and wherein the air-fuel ratio of a mixture supplied to the cylinders is collectively corrected in response to the output from the O2 sensor. On the other hand, there is also known another air-fuel ratio control method, which employs O2 sensors provided for respective cylinder groups and arranged at locations upstream of respective catalytic converters, and wherein air-fuel ratio control amounts are calculated for the respective cylinder groups in a manner independent of the other cylinder groups, to carry out feedback control, based on the calculated control amounts.
The last-mentioned air-fuel ratio control method which controls per cylinder group is superior in air-fuel ratio control accuracy to the aforementioned method, and includes various types, such as a method which controls per bank (right bank and left bank) for a V-type engine, and a method which controls per cylinder group (a first group consisting of #1 and #4 cylinders and a second group consisting of #2 and #3 cylinders) for a straight-type four-cylinder engine.
In the method of controlling per cylinder group, it has been desired to detect deterioration of each of the upstream O2 sensors independently of the other O2 sensors. To meet this desire, an upstream O2 sensor and a downstream O2 sensor may be provided for each cylinder group to detect deterioration of the upstream O2 sensor, which, however, results in a high cost due to an increase in the number of the O2 sensors employed.